It's the most expensive British TV series ever, ­costing £1million an hour to film. But Downton ­Abbey has been a runaway triumph.

The first series of the period saga – which comes to an end on ITV tonight – has attracted more than ­11 million viewers, making it the most successful period drama since Brideshead Revisited. A second series has already been com­missioned.

But the real winners are its young stars, who have all been snapped up by Hollywood on the back of Downton’s success.

Little-known actress Michelle Dockery has been catapulted to stardom by her role as headstrong Lady Mary Crawley, whose torrid liaison with a Turkish diplomat ends in his tragic death.

Michelle – along with her co-stars Dame Maggie Smith, Brendan Coyle, Rob James-Collier and Dan Stevens – have all signed up for the second series. But it is the relative acting ­newcomers who have been propelled to stardom by the show, and many have lined up lucrative roles in big-budget ­blockbusters due next year.

Gorgeous Michelle, 28, will be appearing in Hanna, a thriller about a 14-year-old assassin, with A-lister Eric Bana. Stevens – who plays her handsome suitor Matthew – has just finished filming vampire romance Vamps with Hollywood big-hitters Alicia Silverstone and Sigourney Weaver.

Even the more established stars have seen their careers get a shot in the arm. Hugh Bonneville, who plays the Earl of Grantham, can be seen in current hit Burke and Hare, about the infamous graverobbers. And Elizabeth McGovern – ex-fiancée of Sean Penn – who plays his wife Lady Cora, will star in chilling thriller Drift with Mira Sorvino.

A spokesman for Coyle – the 46-year-old actor who, as lame valet John Bates, has become ­Britain’s most unlikely heartthrob – says: “Brendan was a name ­already but Downton Abbey has ­undoubtedly increased his profile. Offers have been flooding in.”

But ITV insiders fear the programme could become a victim of its own success – thanks to the interest from Hollywood, the stars will undoubtedly have won big payrises for the second series.

For this year’s run, executives – ­desperate to claw back money – have

had to pad out each of the eight 90-minute episodes with up to seven advert breaks.

A source says: “The first series was unbelievably expensive and the second will cost even more. We need to keep our main stars on board – and we will pay over the odds to keep them if we need to.

"But with advertising and ­syndication we are still confident of turning a handsome profit. Downton Abbey is gold dust globally, the sort of thing the Americans in particular love. This is our Avatar, our BBC killer.”

Certainly, the show strays into what is traditionally BBC territory. It has the added benefit of going out after The X Factor, keeping much of the show’s huge audience share. Created by Oscar-winning Gosford Park screenwriter Julian Fellowes, the plot focuses heavily on class.

This is, after all, a man whose 40-acre country estate boasts not just one but three rivers and who insists on calling his many mantelpieces “mantel shelves”.

The former is, apparently, irredeemably naff. But even Fellowes, it seems, needed help this time around, hiring Alastair Bruce, an expert on state and court ritual, as his on-set historical adviser.

Bruce, one of the Queen’s heralds, stalks the set advising on protocol. Should the Earl’s wife shake the hand of a new butler? Certainly not, he insists. So while a footman’s fling with a duke might seem a tad far-fetched, the outfits are certainly authentic.

As Siobhan Finneran, who plays lady’s maid Miss O’Brien, says: “The corsets are legitimate for the period but they are torture. You have to get used to wearing your breasts as earrings.”

Fellowes, 61, is nothing if not a ­magpie. Even his more fanciful plot twists are, it seems, gleaned from the real lives of his aristocratic connections.

For example, the episode of the dead Turk in the bedroom. “That story,” says Fellowes, “came from a splendid house with an incident ­involving one of the male guests. Indeed the whole plot is based on one friend’s history, who will hopefully never recognise himself.”

But Fellowes is more than just an avuncular toff with a rich vault of ­anecdotes. His blue blood masks a keen commercial brain. He targeted ITV for its financial clout: “I think you have to make programmes which will have shelf life, and ­potential on cable, DVD and everything else.”

Despite its starry cast, the real star of Downton Abbey is, of course, the house itself. Set in 6,000 acres of ­glorious Berkshire countryside and boasting a park designed by Capability Brown, Highclere Castle was remodelled in the 19th Century by Sir Charles Barry, who rebuilt the Houses of Parliament.

Home to the 8th Earl and Countess of Carnarvon, it has been used only rarely on television or in cinema – for Jeeves and Wooster and for Stanley Kubrick’s final film Eyes Wide Shut. Sir Andrew Lloyd Webber recently tried to buy it to house his £100million art ­collection but was told firmly: No.

As the countess, whose husband’s family has owned Highclere since 1679, says, when asked how many rooms she has: “I’m not sure. I suppose if you know how many rooms you’ve got, you haven’t got a very big house.”